Photoactivatable coating compositions of the type indicated above are known from, int. al., EP-A-0 952 170. The system disclosed therein is based on dual cure and therefore comprises, besides a (meth)acryloyl-functional polyurethane dispersion, free isocyanate groups which may further react with one or more active hydrogen atoms in the aqueous coating composition. Though high-quality coating layers can be obtained with the known water borne coating compositions, the properties of the aqueous polyurethane dispersions used in the known compositions are still a subject for improvement, especially as far as the appearance of layers containing said dispersions is concerned when applied over substrates with rough surfaces.
Photoactivatable polyurethane resins as such for use in clear coats were already known from DE-C-196 35 447. In this document use is made of a polyurethane- and/or polyester-(meth)acrylate resin with diacrylate monomers as reactive diluents. While the compositions are said to be suitable for use in solvent borne and water borne compositions, in the examples solvent borne compositions only are employed, which are cured by pulsed high-energy ultraviolet radiation.
EP-A-0 965 621 discloses a photoactivatable coating composition which comprises a (meth)acryloyl-functional polyurethane and a photoinitiator such as phenylglyoxylic acids or their esters or salts or derivatives thereof, optionally with other photoinitiators. Though explicit mention is made of the possible use of the claimed coating compositions in water borne systems, in the examples only solvent borne systems are used.